JP7150701B2 - Method for manufacturing high absorber - Google Patents

Description

DESCRIPTION The present invention is a process for the production of superabsorbents comprising polymerization and thermal surface post-crosslinking of a monomer solution, said monomer solution containing at least 0.75% by weight calculated on the total amount of monomers used. It relates to a process in which, before, during or after said thermal surface postcrosslinking, at least 0.09% by weight of aluminum cations, calculated on the total amount of polymer particles used, are added to the polymer particles containing hydroxyphosphonic acid. .

Superabsorbents are used in the manufacture of diapers, tampons, sanitary napkins and other hygiene products, but also as water retention agents in market gardening. Superabsorbents are also called water-absorbing polymers.

The preparation of superabsorbents is described in the research article "Modern Superabsorbent Polymer Technology", F.M. L. Buchholz und A. T. Graham, Wiley-VCH, 1998, p. 71-103.

The properties of the superabsorbent can be adjusted, for example, by the amount of cross-linking agent used. The centrifugal retention capacity (CRC) decreases as the amount of crosslinker increases, and the absorbency under a pressure of 21.0 g/cm ² (AUL 0.3 psi) passes through a maximum value.

To improve performance characteristics such as gel layer permeability (GBP) and absorbency under a pressure of 49.2 g/cm ² (AUL 0.7 psi), superabsorbent particles are generally surface post-crosslinked. thereby increasing the degree of cross-linking of the particle surface, thereby at least partially decoupling the absorbency under pressure of 49.2 g/cm ² (AUL 0.7 psi) and the centrifuge retention capacity (CRC); can be done. This surface postcrosslinking can be carried out in the aqueous gel phase. Preferably, however, the dried, ground and sieved polymer particles (base polymer) are surface-coated with a surface postcrosslinker and thermally surface postcrosslinked. Suitable crosslinkers for that purpose are compounds capable of forming covalent bonds with at least two carboxylate groups of the polymer particles.

WO 2011/113777 A1 describes the production of superabsorbents. At that time, inorganic phosphoric acid or its salt and organic 2-hydroxy acid or its salt are added during or after the polymerization.

WO 2013/144026 A1 describes superabsorbents whose surfaces are complexed with polyvalent metal ions and which comprise phosphonic acid derivatives.

It was an object of the present invention to provide an improved process for producing color-stable superabsorbents exhibiting a high gel layer permeability (GBP).

The aforementioned issues are
a) at least one ethylenically unsaturated monomer having acid groups, which may be at least partially neutralized,
b) at least one cross-linking agent,
c) at least one initiator,
d) optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers described in a), and e) optionally a monomer solution or suspension comprising one or more water-soluble polymers. A method for producing a superabsorbent by liquid polymerization, comprising:
i) polymerizing the monomer solution to obtain a polymer gel,
ii) optionally crushing the resulting polymer gel,
iii) drying the polymer gel,
iv) grinding and classifying the dried polymer gel to obtain polymer particles;
v) thermally surface postcrosslinking the classified polymer particles;
vi) optionally reclassifying the surface postcrosslinked polymer particles,
vii) optionally recycling the polymer particles separated off in step iv) before step iii) and viii) optionally recycling the polymer particles separated off in step vi) before step iii). in a method comprising
Before step i), adding at least 0.75% by weight of hydroxyphosphonic acid or a salt thereof calculated on the total amount of monomers a) used to said monomer solution, and step iv) and step vi) was solved by a method characterized in that at least 0.09% by weight of aluminum cations, calculated on the total amount of polymer particles used, are added to said polymer particles.

As salts of hydroxyphosphonic acids, preference is given to using alkali metal salts, particularly preferably sodium and potassium salts, very particularly preferably sodium salts. All or part of the neutralizable protons of the hydroxyphosphonic acid can then be replaced with any alkali metal cation.

In a preferred embodiment of the invention, the surface postcrosslinked polymer particles are classified again in step vi) and the polymer particles separated off in step vi) are recycled before step iii). The recycled polymer particles preferably have a particle size of less than 250 μm, particularly preferably less than 200 μm, very particularly preferably less than 150 μm.

Preferably 1-hydroxyethylidene-1,1'-diphosphonic acid is used as hydroxyphosphonic acid.

In one preferred embodiment of the invention, aluminum cations are added prior to step v).

Aluminum cations are preferably used as aluminum sulphate.

Preferably at least 0.80% by weight, particularly preferably at least 0.85% by weight, very particularly preferably at least 0.90% by weight of hydroxyphosphonic acid or its salts are monomers, calculated on the total amount of monomers used. Add to solution.

Between step iv) and step vi) preferably at least 0.10% by weight, particularly preferably at least 0.11% by weight, very particularly preferably at least 0% by weight, calculated on the total amount of polymer particles used .12 weight percent aluminum cations are added to the polymer particles.

The present invention achieves high color stability and high gel layer permeability (GBP ) is achieved. The gel layer permeability (GBP) is clearly increased by using sulfate ions as counterions.

Furthermore, it was found that the returned surface postcrosslinked superabsorbent does not absorb the hydroxyphosphonic acid of the monomer solution. When such superabsorbents (fines) are returned to the process, the polymer particles do not exhibit color stability resulting in mottled discoloration. Such mottled discoloration is found to be particularly problematic. Surprisingly, such a discoloration is also observed when the recycled superabsorbent is coated with hydroxyphosphonic acid at the polymer particle stage. In so doing, some polymer particles, especially relatively small polymer particles, may be poorly wetted or not wetted at all by the coating solution.

The following details the production of superabsorbents:
Superabsorbents are produced by polymerizing a monomer solution or suspension in step i) and are usually water-insoluble.

Monomer a) is preferably water-soluble, ie the solubility in water at 23° C. is typically at least 1 g/100 g water, preferably at least 5 g/100 g water, particularly preferably at least 25 g/100 g water, Very particularly preferred is at least 35 g/100 g water.

Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is very particularly preferred.

Other suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a considerable effect on polymerization. It is therefore desirable that the raw materials used have the highest possible purity. It is therefore often advantageous to specifically purify the monomer a). Suitable purification methods are described, for example, in WO 02/055469 (WO 02/055469 A1), WO 03/078378 (WO 03/078378 A1) and WO 2004/035514 (WO 2004/035514 A1). )It is described in. A suitable monomer a) is for example purified according to WO 2004/035514 A1, acrylic acid 99.8460% by weight, acetic acid 0.0950% by weight, water 0.0332% by weight, propion Acrylic acid containing 0.0203% by weight of acid, 0.0001% by weight of furfurals, 0.0001% by weight of maleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl ether.

The proportion of acrylic acid and/or salts thereof in the total amount of monomers a) is preferably at least 50 mol-%, particularly preferably at least 90 mol-% and very particularly preferably at least 95 mol-%.

Monomers a) usually contain polymerization inhibitors, preferably hydroquinone monoether, as storage stabilizers.

The monomer solution preferably contains up to 250 ppm by weight, particularly preferably at most 130 ppm by weight, particularly preferably at most 70 ppm by weight, preferably at least 10 ppm by weight of hydroquinone monoether, based on each unneutralized monomer a) , particularly preferably at least 30 ppm by weight, in particular about 50 ppm by weight. For example, ethylenically unsaturated monomers having acid groups and having a corresponding content of hydroquinone monoether can be used to prepare the monomer solutions.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether (MEHQ) and/or α-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which can be introduced into the polymer chain by radical polymerization, or functional groups which can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts capable of forming coordinate bonds with at least two acid groups of monomer a) are also suitable as crosslinkers b).

Crosslinkers b) are preferably compounds with at least two polymerizable groups which can be introduced into the polymer network by radical polymerization. Suitable crosslinkers b) are for example ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane as described in EP 0530438 A1 triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, EP 0547847 A1, EP 0559476 A1, EP 0559476 A1, EP 0559476 A1 No. 0632068 (EP 0632068 A1), WO 93/21237 (WO 93/21237 A1), WO 03/104299 (WO 03/104299 A1), WO 03/104300 (WO 03/104300 A1), WO 03/104301 A1) and German Patent Application No. 10331450 (DE 10331450 A1); having further ethylenically unsaturated groups in addition to the acrylate groups as described in DE 10331456 A1 and DE 10355401 A1 mixed acrylates or for example DE 19543368 A1, DE 19646484 A1, WO 90/15830 A1 ) and crosslinker mixtures as described in WO 02/032962 A2.

Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane, methylenebismethacrylamide, 15-tuply ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine. be.

Very particularly preferred crosslinkers b) are multiply ethoxylated and/or propoxylated glycerol, as described, for example, in WO 03/104301 A1. They are esterified with acids to form di- or triacrylates. Di- and/or triacrylates of 3- to 10-tuply ethoxylated glycerol are particularly preferred. Di- or triacrylates of 1- to 5-tuply ethoxylated and/or propoxylated glycerol are very particularly preferred. Triacrylates of 3- to 5-tuply ethoxylated and/or propoxylated glycerol are most preferred, and triacrylates of 3-tuply ethoxylated glycerol are particularly preferred.

The amount of crosslinker b) is preferably 0.25 to 1.5% by weight, particularly preferably 0.3 to 1.2% by weight, calculated on the total amount of monomers a) used in each case. Yes, very particularly preferably 0.4 to 0.8% by weight. The centrifugal retention capacity (CRC) decreases as the crosslinker content increases and the absorption capacity under a pressure of 21.0 g/cm ² passes through a maximum value.

As initiator c) any compound that generates radicals under the polymerization conditions may be used, for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodium peroxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite. Preferably, mixtures of thermal and redox initiators are used, for example sodium peroxodisulfate/hydrogen peroxide/ascorbic acid. However, as a reducing component, preferably disodium salt of 2-hydroxy-2-sulfonatoacetic acid or sodium salt of 2-hydroxy-2-sulfinatoacetic acid, disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite A mixture is used. Mixtures of this kind are available as Brueggolite® FF6 and Brueggolite® FF7 (Bruggemann Chemicals; Heilbronn; Germany).

Ethylenically unsaturated monomers d) copolymerizable with ethylenically unsaturated monomers a) having acid groups are for example acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethyl Aminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

As water-soluble polymers e) it is possible to use polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified celluloses such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acid, preferably starches, starch derivatives and modified Cellulose can be used.

Usually an aqueous monomer solution is used. The water content of the monomer solution is preferably 40-75% by weight, particularly preferably 45-70% by weight and very particularly preferably 50-65% by weight. It is also possible to use monomer suspensions, ie monomer solutions with supersaturated monomers a), for example sodium acrylate. As the water content increases, the energy consumption in subsequent drying increases, and as the water content decreases, the heat of polymerization can no longer be dissipated inadequately.

Preferred polymerization inhibitors require dissolved oxygen for optimum action. Dissolved oxygen can therefore be removed from the monomer solution prior to polymerization by inertization, ie by passing an inert gas, preferably nitrogen or carbon dioxide. Preferably, the oxygen content of the monomer solution is reduced prior to polymerization to less than 1 ppm by weight, particularly preferably to less than 0.5 ppm by weight, very particularly preferably to less than 0.1 ppm by weight.

Suitable reactors for the polymerization in step i) are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel that forms during the polymerization of the aqueous monomer solution or suspension is continuously broken up by stirring shafts that are turned against each other, for example as described in WO 2001/038402 A1. do. Polymerization on the belt is described, for example, in DE 3825366 A1 and US Pat. No. 6,241,928. Polymer gels formed when carrying out the polymerization in a belt reactor have to be crushed in a further step ii), for example in an extruder or kneader.

In order to improve drying properties, the crushed polymer gel obtained using a kneader can be further extruded in step ii).

The acid groups of the resulting polymer gel are usually partially neutralized. This neutralization is preferably carried out at the monomer stage. This is usually done by incorporating the neutralizing agent as an aqueous solution, or preferably also as a solid. The degree of neutralization is preferably 25 to 85 mol %, particularly preferably 30 to 80 mol %, very particularly preferably 40 to 75 mol %, wherein a customary neutralizing agent, preferably alkaline Metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates and mixtures thereof can be used. Ammonium salts can also be used instead of alkali metal salts. Particular preference is given to sodium and potassium as alkali metals, but very particular preference is given to sodium hydroxide, sodium carbonate or sodium hydrogen carbonate and mixtures thereof. In this case it is also possible to use solid carbonates and hydrogen carbonates in encapsulated form, which are preferably added to the monomer solution immediately prior to polymerization, to the polymer gel during or after polymerization, and to the drying of the polymer gel. can be introduced before. This encapsulation slows the dissolution and reaction of solid carbonates or bicarbonates such that carbon dioxide is not released until dry and the resulting superabsorbent has a high internal porosity. It is done by coating the surface with an insoluble material (for example with a film-forming polymer, an inert inorganic material or a fusible organic material).

Optionally, a surfactant can be added to the monomer solution before or during polymerization, and the monomer solution is then foamed with inert gas or water vapor or by vigorous stirring before or during polymerization. can be done. Surfactants may be anionic, cationic, zwitterionic or nonionic. It is preferred to use skin-friendly surfactants.

The polymer is then dried in a circulating air belt dryer until the residual moisture content in step iii) is preferably 0.5 to 10% by weight, particularly preferably 1 to 6% by weight, very particularly preferably 1.5 to 4% by weight. Allow the gel to dry. Here, the residual moisture content is determined according to test method No. 1 recommended by EDANA. Measured in accordance with WSP 230.2-05 "Mass Loss Upon Heating". If the residual moisture is too high, the glass transition temperature _Tg of the dried polymer gel will be too low, making further processing difficult. If the residual moisture is too low, the dried polymer gel will be too brittle and the subsequent crushing step will produce an undesirably large amount of polymer particles that are too small in size (fines). The solids content of the polymer gel before drying is preferably 25-90% by weight, particularly preferably 35-70% by weight, very particularly preferably 40-60% by weight. The dried polymer gel is then fractured and, if necessary, coarsely ground.

After this, in step iv) the dried polymer gel is comminuted and classified, the comminution usually using a single- or multi-roll mill, preferably a double- or triple-roll mill, a pin mill, a hammer mill or a vibrating mill. can be done.

The average particle size of the polymer particles separated off as product fraction in step iv) is preferably from 150 to 850 μm, particularly preferably from 250 to 600 μm, very particularly preferably from 300 to 500 μm. The average particle size of the product fraction was measured using test method no. It can be determined according to WSP 220.2-05 "Partikel Size Distribution", in which the average particle size is determined graphically by accumulating and plotting the mass fraction of the sieve fraction. Here, the average particle size refers to the value of the mesh size that becomes 50% by weight when integrated.

The proportion of polymer particles with a particle size greater than 150 μm is preferably at least 90% by weight, particularly preferably at least 95% by weight and very particularly preferably at least 98% by weight.

If the particle size of the polymer particles is too small, the gel layer permeability (GBP) will decrease. A low proportion of excessively small polymer particles (fines) is therefore desirable.

Excessively small polymer particles are therefore usually separated off and recycled to the process, preferably before, during or immediately after the polymerization in step i), i.e. drying of the polymer gel in step iii). done before. Excessively small polymer particles can be moistened with water and/or an aqueous surfactant before or during recycling.

It is also possible to separate off too small polymer particles in a later processing step, for example after surface postcrosslinking in step v) or other coating steps. In this case, the recycled, excessively small polymer particles may be surface postcrosslinked or otherwise coated, for example with pyrogenic silica.

When using a kneading reactor for polymerization, it is preferred to add excessively small polymer particles during the last third of the polymerization in step i). However, it is also possible to incorporate excessively small polymer particles into the polymer gel in a subsequent step ii) of the polymerization reactor, for example in a kneader or extruder.

If too small polymer particles are added too early, for example already in the monomer solution, this will lead to a lower centrifuge retention capacity (CRC) of the resulting polymer particles. However, this can be compensated for, for example, by adjusting the amount of crosslinker b) used.

The proportion of polymer particles with a particle size of at most 850 μm is preferably at least 90% by weight, particularly preferably at least 95% by weight and very particularly preferably at least 98% by weight.

The proportion of polymer particles with a particle size of at most 600 μm is preferably at least 90% by weight, particularly preferably at least 95% by weight and very particularly preferably at least 98% by weight.

If the particle size of the polymer particles is too large, the swelling rate will decrease. It is therefore desirable that the proportion of excessively large polymer particles be low as well. Excessively large polymer particles are therefore usually separated off and returned to the grinding.

In order to further improve the properties, the polymer particles can be thermally surface postcrosslinked in step v). Suitable surface postcrosslinkers are compounds containing groups capable of forming covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds are described, for example, in EP 0083022 A2, EP 0543303 A1 and EP 0937736 A2 polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in DE 33 14 019 A1, DE 33 14 019 A1, DE 35 23 617 A1 3523617 A1) and difunctional or polyfunctional alcohols as described in EP 0 450 922 A2 (EP 0 450 922 A2) or DE 10 204 938 A1 (DE 10 204 938 A1). ) and β-hydroxyalkylamides as described in US Pat. No. 6,239,230 (US 6,239,230).

Furthermore, DE 40 20 780 C1 describes cyclic carbonates and DE 198 07 502 A1 describes 2-oxazolidinones and their derivatives such as 2- Hydroxyethyl-2-oxazolidinone is described in DE 19807992 C1, bis-2-oxazolidinone and poly-2-oxazolidinone are described in German Patent Application 19854573 (DE 19854573 A1) for 2-oxotetrahydro-1,3-oxazines and their derivatives, DE 19854574 A1 for N-acyl-2-oxazolidinones, German Patent Application DE 102 04 937 A1 describes cyclic ureas, DE 103 34 584 A1 describes bicyclic amide acetals, EP 1 199 327 A1 ( EP 1199327 A2) describes oxetanes and cyclic ureas, and WO 03/031482 A1 describes morpholine-2,3-diones and their derivatives as suitable surface postcrosslinkers. ing.

Preferred surface postcrosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin, and mixtures composed of propylene glycol and 1,4-butanediol.

Very particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1,3-propanediol.

Furthermore, it is also possible to use surface postcrosslinkers with additional polymerisable ethylenically unsaturated groups, as described in DE 37 13 601 A1 (DE 37 13 601 A1).

The amount of surface postcrosslinker is preferably 0.001 to 3% by weight, particularly preferably 0.02 to 1% by weight and very particularly preferably 0.05 to 0.05% by weight, each based on the polymer particles. 2% by weight.

Surface postcrosslinking is usually carried out by spraying a solution of the surface postcrosslinker onto the dry polymer particles. This spraying is followed by surface postcrosslinking of the polymer particles coated with the surface postcrosslinker and drying, which surface postcrosslinking reaction can take place not only before drying but also during drying.

The spraying of the solution of the surface postcrosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and blade mixers. Particular preference is given to horizontal mixers, for example blade mixers, very particular preference to vertical mixers. The distinction between horizontal and vertical mixers is made by the mounting of the mixing shaft, ie horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft.適したミキサーは、例えばＨｏｒｉｚｏｎｔａｌｅ Ｐｆｌｕｇｓｃｈａｒ（登録商標）ミキサー（Ｇｅｂｒ．Ｌｏｅｄｉｇｅ Ｍａｓｃｈｉｎｅｎｂａｕ ＧｍｂＨ；パーダーボルン；ドイツ）、Ｖｒｉｅｃｏ－Ｎａｕｔａ Ｃｏｎｔｉｎｕｏｕｓ Ｍｉｘｅｒ（Ｈｏｓｏｋａｗａ Ｍｉｃｒｏｎ ＢＶ；ドゥーティンヘム；オランダ）、Ｐｒｏｃｅｓｓａｌｌ Ｍｉｘｍｉｌｌ Ｍｉｘｅｒ（Ｐｒｏｃｅｓｓａｌｌ Ｉｎｃｏｒｐｏｒａｔｅｄ；シンシナティUSA) and Schugi Flexomix® (Hosokawa Micron BV; Dutinchem; The Netherlands). However, it is also possible to spray the surface postcrosslinker solution in a fluidized bed.

Surface postcrosslinkers are typically used as aqueous solutions. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by the content of non-aqueous solvent and the total amount of solvent.

If only water is used as solvent, it is preferred to add a surfactant. This improves the wetting behavior and reduces the tendency to clump. However, preference is given to using solvent mixtures such as isopropanol/water, 1,3 propanediol/water and propylene glycol/water, the mixing weight ratio preferably being from 20:80 to 40:60.

Surface postcrosslinking is preferably carried out in contact dryers, particularly preferably paddle dryers, very particularly preferably disk dryers. Suitable drying devices are, for example, the Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Rheingarten; Germany), the Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Rheingarten; Germany), the Holo ® dryers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Furthermore, a fluidized bed dryer can also be used.

Surface postcrosslinking can be carried out in the mixer itself by heating the jacket or blowing in hot air. Subsequent drying devices such as box dryers, rotary tube furnaces or heatable screws are likewise suitable. It is particularly advantageous to carry out the surface postcrosslinking by mixing and heat in a fluid bed dryer.

Preferred reaction temperatures are in the range from 100 to 250°C, preferably in the range from 120 to 220°C, particularly preferably in the range from 130 to 210°C, very particularly preferably in the range from 150 to 200°C. Preferred residence times at this temperature are preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes and usually at most 60 minutes.

In one preferred embodiment of the invention, the polymer particles are cooled after the surface postcrosslinking. This cooling preferably takes place in a contact cooling system, particularly preferably a paddle cooling system, very particularly preferably a disc cooling system. Suitable cooling devices are, for example, the Hosokawa Bepex® Horizontal Paddle Cooler (Hosokawa Micron GmbH; Rheingarten; Germany), the Hosokawa Bepex® Disc Cooler (Hosokawa Micron GmbH; Rheingarten; Germany), the Holo - Flite® coolers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Coolers (NARA Machinery Europe; Frechen; Germany). Furthermore, a fluidized bed type cooling device can also be used.

In the cooling device, the polymer particles are preferably cooled to 40-90°C, particularly preferably 45-80°C, very particularly preferably 50-70°C.

Subsequently, the surface-postcrosslinked polymer particles can be classified again, with excessively small and/or excessively large polymer particles being separated off and recycled to the process.

In order to further improve the properties, the surface-postcrosslinked polymer particles may be coated or post-moisturized.

Subsequent moistening is preferably carried out at from 40 to 120.degree. C., particularly preferably from 50 to 110.degree. C., very particularly preferably from 60 to 100.degree. At excessively low temperatures the polymer particles tend to agglomerate, and at relatively high temperatures the water already evaporates significantly. The amount of water used for post-moistening is preferably 1 to 10% by weight, particularly preferably 2 to 8% by weight and very particularly preferably 3 to 5% by weight. This post-moistening increases the mechanical stability of the polymer particles and reduces their tendency to charge. This post-humidification is preferably carried out in a cooling device after the thermal surface postcrosslinking.

Suitable coatings for improving swelling rate and gel layer permeability (GBP) are, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. . Coatings suitable for binding dust are, for example, polyols. Suitable coatings for counteracting the undesirable caking tendency of polymer particles are eg pyrogenic silicas such as Aerosil® 200 and surfactants such as Span® 20. Coatings suitable for binding dust, reducing tendency to caking and improving mechanical stability are polymer dispersions such as those described in EP 0703265 B1 and US Pat. Waxes as described in the specification (US 5840321).

A further subject of the present invention is a superabsorbent produced by the method according to the invention, said superabsorbent particles having a centrifugal retention capacity of at least 25 g/g under a pressure of 49.2 g/cm ² . at least 15 g/g, a gel layer permeability of at least 60 darcy, and said superabsorbent particles have an L values, a values less than 7.0 and b values less than 14.5.

The superabsorbent particles according to the invention exhibit high color stability even in mixtures with pulp, such as is normally present in diapers. In contrast, the superabsorbent particles, to which the color stabilizer has been applied only to their surface, show a distinctly lower color stability in mixtures with pulp.

The superabsorbent particles produced by the process according to the invention preferably have a centrifuge retention capacity (CRC) of at least 26 g/g, particularly preferably at least 27 g/g, very particularly preferably at least 28 g/g. be. The centrifuge retention capacity (CRC) of the superabsorbent particles is typically less than 60 g/g.

The superabsorbent particles produced by the process according to the invention preferably have an absorbency under a pressure of 49.2 g/cm ² (AUL 0.7 psi) of at least 16 g/g, particularly preferably of at least 17 g/g. and very particularly preferably at least 18 g/g. The absorbency under pressure of 49.2 g/cm ² (AUL 0.7 psi) of the superabsorbent particles is typically less than 35 g/g.

The superabsorbent particles produced by the process according to the invention preferably have a gel layer permeability (GBP) of at least 65 darcy, particularly preferably at least 70 darcy, very particularly preferably at least 75 darcy. The gel layer permeability (GBP) of the superabsorbent particles is typically less than 400 darcy.

The superabsorbent particles produced by the process according to the invention preferably have an L value of at least 68, particularly preferably of at least 69, very particularly preferably of at least 70 after storage for 14 days at a temperature of 70° C. and a relative humidity of 80%. have.

The superabsorbent particles produced by the process according to the invention preferably have an a value of less than 6.6 and a b value of less than 14.0 after storage for 14 days at a temperature of 70° C. and a relative humidity of 80%, particularly preferably It has an a value of less than 6.2 and a b value of less than 13.5, very particularly preferably an a value of less than 5.8 and a b value of less than 13.0.

Method:
The standard test methods identified below with the designation "WSP" are the "Worldwide Strategic Partners" EDANA (Avenue Eugene Plasky, 157, 1030 Brussels, Belgium, www.edana.org) and INDA (1100 Crescent Green, Suite 115, Cary, North Carolina 27518, USA, www.inda.org), "Standard Test Methods for the Nonwovens Industry," 2005 edition. The publication is available from both EDANA and INDA.

Unless otherwise stated, measurements should be made at an ambient temperature of 23±2° C. and a relative air humidity of 50±10%. The water-absorbing polymer particles are thoroughly mixed prior to measurement.

Centrifuge Retention Capacity
Centrifuge Retention Capacity (CRC) was measured using test method no. Measured according to WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation".

Absorption under Load of 21.0 g/cm ²
Absorbency (AUL 0.3 psi) under a pressure of 21.0 g/cm ² of the water-absorbent polymer particles was measured using Test Method No. 2 recommended by EDANA. Measure according to WSP 242.2-05 "Absorption Under Pressure, Gravimetric Determination".

Absorption under Load of 49.2 g/cm ²
The absorbency under a pressure of 49.2 g/cm ² (AUL 0.7 psi) was measured using test method no. Measured as in WSP 242.2-05 "Absorption Under Pressure, Gravimetric Determination", with a pressure of 49.2 g/cm ² (AUL 0.3 psi) instead of a pressure of 21.0 g/cm ² (AUL 0.3 psi). 7 psi).

Gel Bed Permeability
The gel layer permeability (GBP) of the swollen gel layer under a pressure load of 0.3 psi (2070 Pa) was measured according to US 2005/0256757 (paragraphs [0061] and [0075 ]), it is determined as the gel layer permeability of the swollen gel layer of the water-absorbing polymer particles.

CIE Chromaticity (L, a, b)
Colorimetry was performed using a "LabScan XE S/N LX17309" colorimeter (HunterLab, Reston, USA) according to the CIELAB method (Hunterlab, Vol. 8, 1996, No. 7, pp. 1-4). according to Colors are then represented by coordinates L, a and b in a three-dimensional system. L stands for lightness, where L=0 means black and L=100 means white. The a and b values represent the color position on the red/green or yellow/blue color axis, where positive a represents red, negative a represents green, and positive b represents yellow. and a negative b represents a blue color. Calculate the HC60 value by the formula HC60=L-3b.

This colorimetric determination is consistent with the tristimulus method according to DIN 5033-6.

Example Example 1
406.8 g of acrylic acid, 4271.4 g of an aqueous sodium acrylate solution (37.3% by weight concentration), 130.4 g of water and 5.86 g of tri-ethoxylated glycerin triacrylate (about 85% by weight concentration), The monomer solution, deoxygenated with nitrogen gas for 30 minutes, is polymerized in a polymerization reactor of the type LUK 8.0K2 equipped with two parallel shafts (Coperion Werner & Pfleiderer GmbH & Co. KG; Stuttgart, Germany). let me This monomer solution was further mixed with 77.29 g of an aqueous 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt solution (concentration 20% by weight). Polymerization was carried out by adding 15.84 g of an aqueous sodium peroxodisulfate solution (15% by weight concentration), 1.41 g of an aqueous hydrogen peroxide solution (1% by weight concentration) and 91.12 g of an aqueous ascorbic acid solution (0.5% by weight concentration). got it started. Polymerization was complete after about 30 minutes. The resulting polymer gel was comminuted three times using a commercial mincer with a 6 mm perforated plate and dried in a laboratory drying cabinet at 175° C. for 90 minutes. The dried polymer was ground and sieved to a particle size of 150-710 μm. The base polymer thus produced exhibited a Centrifuge Retention Capacity (CRC) of 38 g/g.

1000 g of the base polymer for surface postcrosslinking in a plowshare mixer type M5R (Gebrueder Loedige Maschinenbau GmbH; Paderborn; Germany) at 23° C. and a shaft speed of 200 rpm using two two-component spray nozzles as follows: Coated with solution:
Solution I: 0.50 g of N-(2-hydroxyethyl)-2-oxazolidinone
0.50 g of 1,3-propanediol
33.10 g of aluminum sulfate aqueous solution (concentration 26.8% by weight)
Solution II: 10.0 g isopropanol.

After spraying the two solutions the temperature was raised to 195° C. and the reaction mixture was held at this temperature and a shaft speed of 60 rpm for 75 minutes. Subsequently, the reaction mixture was sieved again at 23° C. to a particle size of 150-710 μm.

The obtained superabsorbent particles were analyzed. To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 1.

Example 2 (comparative example)
As in Example 1. Instead of 77.29 g of an aqueous 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt solution (concentration 20% by weight), an aqueous 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt solution (concentration 20% by weight) was added to the monomer solution. % by weight) was mixed.

The obtained superabsorbent particles were analyzed. To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 1.

Example 3 (comparative example)
As in Example 1. Only 11.10 g of aqueous aluminum sulphate solution (26.8 wt. % concentration) were used instead of 42.90 g of aqueous aluminum sulphate solution (26.8 wt. % strength) for the surface postcrosslinking.

The obtained superabsorbent particles were analyzed. To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 1.

Example 4 (comparative example)
As in Example 1. The monomer solution did not contain 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt. Instead, 15.46 g of 2-hydroxy-2-sulfonatoacetic acid disodium salt was mixed into the monomer solution.

During polymerization, the resulting polymer gel wrapped around the shaft of the polymerization reactor. I had to stop the experiment.

Example 5 (comparative example)
As in Example 1. The monomer solution did not contain 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt.

The obtained superabsorbent particles were analyzed. To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 1.

Example 6
As in Example 1. No aluminum sulfate was used for surface postcrosslinking. Instead, 80.8 g of an aqueous aluminum trilactate solution (concentration 18.9% by weight) was used.

The obtained superabsorbent particles were analyzed. To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 1.

SXL: surface postcrosslinking Cublen: 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt Blancolen: 2-hydroxy-2-sulfonatoacetic acid disodium salt
^* ) Comparative experiment
^** ) Impossible, polymerization interrupted.

The results of Examples 1-3 demonstrate that high absorbers exhibiting high color stability and high gel layer permeability (GBP) cannot be obtained without high amounts of hydroxyphosphonic acid (Cublen) and high amounts of aluminum cations. showing.

Example 4 shows that the amount of 2-hydroxysulfonic acid (Blancolen) in the monomer solution required for color stabilization interferes with polymerization.

Examples 1 and 6 show the advantage of sulfate over lactate as a counterion.

Example 7
A 43.0% strength by weight acrylic acid/sodium acrylate solution was prepared by mixing water, aqueous caustic soda solution and acrylic acid. The degree of neutralization of the monomer solution so prepared was 72.0 mol %.

The monomer solution was degassed with nitrogen at about 23°C. Tri-ethoxylated glycerin triacrylate (concentration about 85% by weight) was used as the polyethylenically unsaturated crosslinker. The amount of 3-tuply ethoxylated glycerin triacrylate used was 0.16% by weight calculated on the total amount of acrylic acid used.

Additionally, a 20% by weight aqueous solution of 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt was added to the monomer solution. The amount of 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt used was 0.30% by weight calculated based on the total amount of acrylic acid used.

To initiate radical polymerization, the following ingredients were used: hydrogen peroxide (0.002 wt% of a 1.0 wt% aqueous solution), sodium peroxodisulfate (0.15 wt% of a 15 wt% aqueous solution) and Ascorbic acid (0.01% by weight of a 0.5% by weight aqueous solution). Weight percents are calculated based on the total amount of acrylic acid used.

The individual components were continuously metered into a List ORP type 10 kneader reactor (ListAG, Arisdorf, Switzerland). The throughput of the monomer solution was 40 kg/h.

The reaction solution had a temperature of about 23° C. at the time of delivery. The residence time of this reaction mixture in the reactor was approximately 15 minutes. After polymerization and gel crushing, 1 kg each of the aqueous polymer gel was applied to a drying plate with a sieve mesh bottom of 300 μm and dried at 175° C. for 60 minutes in a circulating air drying cabinet. The dried polymer gel was pulverized with a roll mill and classified.

To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 2.

Example 8
As in Example 7. In the first third of the reactor, a further 0.86 kg/h of superabsorbent with a particle size of less than 150 μm was added. The superabsorbent particles added were thermally surface postcrosslinked and did not contain 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt.

To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 2.

Example 9
As in Example 7. In the first third of the reactor, a further 1.72 kg/h of superabsorbent with a particle size of less than 150 μm was added. The superabsorbent particles added were thermally surface postcrosslinked and did not contain 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt.

To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 2.

example 10
As in Example 7. In the first third of the reactor a further 2.58 kg/h of superabsorbent with a particle size of less than 150 μm was added. The superabsorbent particles added were thermally surface postcrosslinked and did not contain 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt.

To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 2.

Example 11
As in Example 7. In the first third of the reactor a further 3.44 kg/h of superabsorbent with a particle size of less than 150 μm was added. The superabsorbent particles added were thermally surface postcrosslinked and did not contain 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt.

To examine color stability, the superabsorbent particles were stored at 70° C. and 80% relative humidity for 14 days. The results are summarized in Table 2.

No visible mottled discoloration +
Slight visible mottled discoloration -
There is a distinctly visible mottled discoloration --
There is a very distinct visible mottled discoloration---.

The results of Examples 7-11 show that due to thermal surface postcrosslinking, the superabsorbent particles no longer absorb 1-hydroxyethylidene-1,1-diphosphonic acid disodium salt and become discolored during storage. there is The mottled discoloration increases with the amount of superabsorbent added.

Claims (14)

a) at least one ethylenically unsaturated monomer having acid groups, which may be at least partially neutralized,
b) at least one cross-linking agent,
c) at least one initiator,
d) optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers described in a), and e) optionally a monomer solution or suspension comprising one or more water-soluble polymers. A method for producing superabsorbent particles by polymerizing a liquid, comprising:
i) polymerizing the monomer solution to obtain a polymer gel,
ii) optionally crushing the resulting polymer gel,
iii) drying the polymer gel,
iv) grinding and classifying the dried polymer gel to obtain polymer particles;
v) thermally surface postcrosslinking the classified polymer particles;
vi) optionally reclassifying the surface postcrosslinked polymer particles,
vii) optionally recycling the polymer particles separated off in step iv) before step iii) and viii) optionally recycling the polymer particles separated off in step vi) before step iii). in a method comprising
Before step i), adding at least 0.75% by weight of hydroxyphosphonic acid or a salt thereof calculated on the total amount of monomers a) used to said monomer solution, and step iv) and step vi) , wherein at least 0.09% by weight of aluminum cations are added to said polymer particles, calculated on the total amount of polymer particles used, and said aluminum cations are used as aluminum sulphate . 2. Process according to claim 1, characterized in that the polymer particles separated off in step vi) and recycled before step iii) have a particle size of less than 150 [mu]m. 3. Process according to claim 1, characterized in that 1-hydroxyethylidene-1,1'-diphosphonic acid is used as hydroxyphosphonic acid. 4. Process according to any one of claims 1 to 3, characterized in that said aluminum cations are added before step v). from claim 1, characterized in that at least 0.8% by weight of hydroxyphosphonic acid or a salt thereof, calculated on the total amount of monomers a) used, is added to the monomer solution before step i) 4. A method according to any one of the preceding claims. from claim 1, characterized in that at least 0.85% by weight of hydroxyphosphonic acid or a salt thereof, calculated on the total amount of monomers a) used, is added to the monomer solution before step i) 5. A method according to any one of the preceding items. 7. Any one of claims 1 to 6, characterized in that at least 0.1% by weight of aluminum cations, calculated on the total amount of polymer particles used, are added to the polymer particles after step iv). The method described in the section. 8. Any one of claims 1 to 7, characterized in that at least 0.11% by weight of aluminum cations, calculated on the total amount of polymer particles used, are added to the polymer particles after step iv). The method described in the section. Superabsorbent particles obtainable by the method according to any one of claims 1 to 8 , said superabsorbent particles having a centrifuge retention capacity of at least 25 g/g and 49.2 g/cm ² . of at least 15 g/g under a pressure of at least 15 g/g, a gel layer permeability of at least 60 darcy, and said superabsorbent particles are at least Superabsorbent particles having an L value of 67, an a value of less than 7.0 and a b value of less than 14.5. 10. The superabsorbent particles of claim 9 , wherein said centrifuge retention capacity is at least 28 g/g. 11. Superabsorbent particles according to claim 9 or 10 , wherein the absorbency under a pressure of 49.2 g/cm< ² > is at least 18 g/g. 12. Superabsorbent particles according to any one of claims 9 to 11 , wherein the gel layer permeability is at least 75 Darcy. 13. Superabsorbent particles according to any one of claims 9 to 12 , having an L value of at least 70 after storage for 14 days at a temperature of 70<0>C and a relative humidity of 80%. Superabsorbent particles according to any one of claims 9 to 13 , having an a value of less than 5.8 and a b value of less than 13.0 after storage for 14 days at a temperature of 70°C and a relative humidity of 80%. .